1. Technical Field
It is common practice in electronic circuits to amplify or attenuate a received input signal using a variable gain amplifier so as to, for example, ensure that further circuit elements receive the maximum value of input signal to maximise the signal to noise ratio of the received signal. The provision of a variable gain amplifier to receive a variable amplitude input signal prior to further processing of the amplified input signal by other circuit elements is typical in audio and radio frequency receivers.
2. Related Technology
In such electronic circuits the gain of the variable in amplifier is set by using a number of switches to select one or more input resistors to set the required gain. The switches are implemented as transistors. Depending on the input signal to the variable gain amplifier, the resistance of the transistors used to implement the switches can vary as a function of the input signal, this variation in resistance of the transistor having an effect on the overall gain of the amplifier. As the input signal is typically a non-constant amplitude signal, the resistance of the transistors can vary as the input signal varies and therefore the performance of the variable gain amplifier can vary. This non-linear behaviour of the variable in amplifier in certain circumstances is particularly undesirable when very high quality performance is required, since the non-linear effects of the transistor switches can become dominant and lead to harmonic distortions on the output signal. It would therefore be advantageous to provide a variable gain amplifier stage in which the effect of the non-linearity of the transistor switches was at least reduced.